Increasingly there is a need for motion picture producers and exhibitors to differentiate their product at motion picture theatre multiplexes from that of competitors and to differentiate the theatre experience from that which customers can obtain at home. One approach is to provide images that are larger, sharper and brighter than what viewers can experience elsewhere.
A number of attempts have been made over the years to improve the performance of film based projectors by tiling multiple projectors together (e.g. Cinerama in the 1950s) or by using a larger 5 perforation 70 mm film format (e.g. Todd AO or Cinemascope). The applicant, IMAX Corporation, successfully developed a higher performance motion picture system using a 15 perforation 70 mm film format; enabled by a rolling loop film transportation mechanism.
Another approach to differentiate the performance of film based projectors is to exhibit 3D motion pictures. This approach has been commercialized by various organizations including the applicant over the years. Typically 3D presentation requires two filmstrips, one for each eye, and two separate projectors to display the images contained on the filmstrips. Sometimes it may be desirable to convert such a system so that a standard 2D motion picture can be shown, and in the case of a two projector system it is straight forward; one projector can be switched off while the other is used. We shall see that the invention disclosed below has the benefit of improving performance by using the second projector in 2D operation rather than letting it sit idle.
An emerging trend within the motion picture industry is to replace standard film based projection with state of the art electronic projectors for a variety of reasons including cost savings in motion picture distribution, and presentation of live events in real time. A disadvantage of current electronic projectors is that they are limited in resolution and light output required for large immersive screens. This is mainly due to manufacturing economics and the current emphasis on electronic projectors that propose to compete with standard 35 mm film based projection only. One approach to deal with the resolution and light output limits of electronic projectors is to tile or combine the output of multiple separate projectors to form one large composite image at the display screen surface. A number of patents have been granted discussing various methods of tiling or stitching together the images of separate electronic projectors including:
U.S. Pat. No. 5,956,000 discloses a method of combining N projectors together to form a composite image where the sub-images overlap and where the overlap areas are modulated to compensate for the increased brightness in those regions. The sub images are also corrected for misalignments.
U.S. Pat. No. 6,115,022 involves the use of a three-dimensional array of smoothing factors that is applied to the blending of overlapped image seams as well as to other composite image artifacts.
U.S. Pat. No. 6,456,339 discloses a method of generating a projector to screen map by combining the results of a camera to screen mapping with a camera to projector mapping. The projector to screen map is used to produce a pixel correcting function that in turn is used to warp images to correct for misalignments and to correct for illuminance and color artifacts in the region of the screen where the images overlap.
U.S. Pat. No. 6,222,593 describes a multiple projector system that tiles images together to achieve a high resolution display. Images are captured from a camera and parameters are calculated to permit warping the output of each of the projectors via analytic expressions.
U.S. Pat. Nos. 6,568,816 and 6,760,075 describes a projection system, which has a single light source that supplies light to multiple projection heads whose output are sub-images that overlap to form composite images. The single light source ensures that colorimetery matching problems between the sub-images are eliminated.
U.S. Pat. No. 6,570,623 discloses the use of blending frames located between the projection lenses and the display screen to control the brightness of the images in the overlapping region, and further discloses the use of an adaptive technique using a camera based iterative algorithm to fine tune the blending of overlapped images.
U.S. Pat. No. 6,771,272 discloses a graphics system comprising pixel calculation units and a sample buffer that is used to correct for display non-uniformities, such as seam overlap brightness by appropriately scaling pixel values prior to projection.
U.S. Pat. No. 6,733,138 describes a system of forming a mosaic image from multiple projectors by projecting registration images from each to form a union registration image. This registration image is then used to generate a projective matrix, which is used to warp individual source images to achieve a unified composite image. The brightness from each of the projectors is weighted in the overlap regions to minimize seam visibility.
U.S. Pat. No. 6,804,406 describes a composite image display method using display to screen and screen to camera spatial transformation functions as well as a spatial luminance transfer function to pre-warp image segments prior to projection. An inverse of the spatial luminance function is used to blend colors in the tiled composite image.
U.S. Pat. No. 6,814,448 discloses a composite image display system which uses test images and means of sensing to determine correction data that is used to provide for a uniform level of illumination, overlap regions included, across the entire display screen surface.
All of these tiling techniques use various combinations of optical and electronic image correction to ensure that the overlapped region is indistinguishable from non-overlapped regions. Electronic image correction sacrifices the number of bits available to display images (bit depth) because some of the available image bits are used to correct for non-uniformities in brightness and color. In order to correct for brightness, color mismatches and spatial misalignments of pixels between the projectors a calibration technique, which measures the image on the screen to determine the required correction must be employed.
Conventional methods to achieve tiling require warping of images from every projector in the system. Each projector carries its own set of distortions that need to be eliminated in order to prevent artifacts near or within the overlap region. The removal of all distortions requires a mapping onto absolute screen coordinates, which is done through analytic expressions.
In the process of equalizing brightness and color between the two projectors, the output from each color channel must be adjusted. This adjustment is subtractive and leads to a lower light output of the combined system. These displays that use tiling must be frequently recalibrated primarily due to the reduction in brightness or changes in color that occur as the lamps age.
As well, these patents listed above do not address the unique requirements of projecting 3D stereoscopic motion picture images. Foremost 3D projection requires two separate and coded channels of image data to be projected, one for each eye's (left and right) point of view. In a tiled system the only way to achieve separate left and right eye images without modification of the system is to multiplex left and right eye images in time. As such the display duration of each frame is halved with the first portion devoted to displaying left eye images and the second portion for displaying right eye images. While this approach is possible, in one implementation, it requires expensive alternate eye shutter glasses to be worn by audience members. The need for alternate eye glasses can be eliminated with the use of a fast acting polarization converting element to switch the polarization of images for the right and left eyes thus allowing passive polarizing glasses to be worn by the audience, see for example, U.S. Pat. No. 4,281,341. Whether alternate eye shutter glasses are used or a fast acting polarizer is employed, time multiplexing the left and right eye images sacrifices brightness. As well, these methods place a higher demand on the electronic projectors to show content at faster frame rates and results in a reduced bit depth of the projected images.
There are also alternative approaches to project 3D in a tiled projection system that would require modification to placement of images on the screen. In the case of a two projector system, this would require that the output of the two projectors be fully overlapped. A passive 3D technique may then be used (polarizers or color filters) to separate left and right eye images. However, converting a system that requires images to be tiled for 2D operation and overlapped for 3D operation within a short time period time between 2D and 3D motion picture screenings would be complex and cost prohibitive.
A preferred approach for combining the output of two or more projectors used for 3D and 2D presentations is to completely overlap the two images. When images are completely superimposed, differences in brightness and color between the two projectors do not appear as local discontinuities that are readily detectable by the human eye. As such, a completely superimposed image does not suffer the loss in image bit depth and brightness incurred in a tiled display to achieve the required uniformity and does not require calibration to ensure overlapped and non-overlapped regions are indistinguishable. In a fully overlapped system the only calibration that is required is the measurement of the spatial distortions that cause pixel misalignments among the pixels projected from different projectors. A projection system that superimposes images is thus more robust due to insensitivity to changes in brightness and color of the images that occur as the system is used.
The following patents discuss various embodiments of fully overlapped component projectors achieved by electronically warping the image data. U.S. Pat. No. 6,456,339. In one embodiment of this patent, the images of two projectors having a small pixel fill factor are completely overlapped to produce a super resolution display. U.S. Pat. No. 6,222,593 describes an embodiment where their warping system is used to superimpose two images that may be used to increase 2D light levels or may be used for 3D applications.
U.S. Patent Application No. 2004/0239885 discloses a super resolution composition method that uses a derived projector correspondence map to a target surface. All component images are warped to the target surface and then an algorithm working in the spatial frequency domain optimizes the image quality. This optimization process depends on the image being displayed and is iterative malting it unsuitable for real time motion picture projection.
The following patents describe methods for increasing the resolution of a display by superimposing with a half pixel offset between the component images without warping the images electronically. Offset may be defined to be a vector displacement with two orthogonal components.
U.S. Pat. No. 5,490,009 discloses a method of enhancing the horizontal and/or vertical resolution of a display device by simultaneously combining the output of two or more offset spatial light modulators.
U.S. Pat. No. 6,222,593 is primarily focused on methods for tiling, but does mention the possibility of superposition of images to increase light levels and to allow the system to be used for 3D presentations.
U.S. Pat. No. 6,231,189 discloses a dual polarization optical projection system capable of 2D and 3D presentations in which separate component images are combined prior to projection through a single projection lens. The resulting images are fully overlapped on the projection screen and can be used to increase the brightness of the display, increase the resolution of the display by imposing a fixed offset of one image relative to the other of less than one pixel, or project stereoscopic images using the orthogonal polarization of the component images to distinguish left and right eye images.
Other patents, such as for example, U.S. Pat. Nos. 6,231,189 and 5,490,009, disclose methods to achieve higher brightness and resolution by superimposing projectors with a fixed sub-pixel offset relative to each other. In order to achieve the fixed offset when projecting on a curved screen, the images must be combined through a single projection lens as disclosed in U.S. Pat. No. 6,231,189. This negates the possibility of using off-the-shelf projectors. In addition, there are considerable challenges involved to mechanically register pixels with a fixed sub-pixel offset and maintain this offset over repeated use. In particular, when illuminating large screens the amount of light that must travel through the system results in thermal cycling that makes pixel registration more challenging.
To overcome challenges of maintaining a fixed sub-pixel registration required when combining multiple projectors to enhance brightness and increase resolution, certain patents or published patent applications such as, for example, U.S. Patent Application No. 2004/0239885 and U.S. Pat. Nos. 6,456,339, 6,814,448, and 6,570,623 disclose methods of image warping. These warping methods use a calibration method to measure the spatial misalignments between different projectors. These calibration methods calculate a correspondence map between the projectors and a screen co-ordinate system for warping the image data to correct for geometric distortions. The distortions result from optical or projection point differences among the projectors. The calibration methods disclosed work on the premise of being able to calculate absolute screen positions. Absolute screen positions are required to correct distortions caused by projection points that deviate significantly from normal incidence relative to the screen or where the intended application is sensitive to distortions. In order to convert images taken by a camera to absolute screen positions, the distortion of the camera and the relationship of the camera to the screen must be known. In these systems both images are warped to the absolute screen coordinates. If multiple cameras are used, then the calibration of the cameras to the screen must be extremely accurate in order to ensure correct warping of the images to achieve pixel registration. As disclosed in the prior art, this requires moving a physical test target across the screen surface. In a large cinema projection system this method of calibration is not practical.
The above patents do not address the needs that a motion picture cinema projection system must fulfill to be successful against competing display technologies. In particular, systems that requite the determination of absolute screen coordinates to superimpose images are unnecessarily complex and impractical to implement in a cinema theatre environment. They do not take advantage of the fact that, in a theatre environment, the image from a projector has relatively low distortion and may be projected essentially without modification onto the screen. This arises from the fact that the optical axis of the projection system is tear normal incidence to the screen in a typical theatre environment. In addition, an immersive cinematic experience requires a large field of view that can't be seen all at once. In this situation, distortions occur gradually relative to the viewer's gaze and are not noticeable.
There are additional requirements that are not easily met by existing art when the cinema projector must show both 2D and 3D presentations or when one presentation is a mixture of 2D and 3D formats. In some cases a custom projection system must be designed. In other cases, one must resort to using expensive shutter glasses or incur the light loss of 3D methods that use time multiplexing to distinguish left and right eye images.
Existing art does not take advantage of the different requirements of 2D and 3D presentation and the display properties required for an immersive experience. There is a difference between optimal brightness for 2D compared to 3D projection. In 3D projection a trade-off exists between brightness and perceived cross-talk between left and right eyes. Cross-talk occurs when a right-eye image leaks into the left eye or vise versa. This ghosting artifact is more apparent when the semen brightness is increased. As a result, the optimal brightness for 3D projection is generally lower than that required for 2D projection.
In addition to providing enhancements to the two modes of presentation, a successful system must also provide: a high quality of presentation in these modes; be cost effective; be easy to set up and calibrate; allow for a quick conversion from one mode to the other, and be easy to maintain.
The needs described above require a unique and optimal combination of the physical arrangement, calibration and mapping of the component projectors. This combination of elements is the subject of this patent and will be discussed in more detail below.